X-ray photoelectron spectroscopy study of the germanate anomaly in the Na₂O-GeO₂ system

Photoelectron spectra of the O 1s and Ge 3d levels have been obtained from a series of Na₂O-GeO₂ glasses of general formula xNa₂O. (1-x) GeO₂ (x=0·125, 0·175, 0·2 and 0·3) and from powders of quartz-like GeO₂ and rutile-like GeO₂. The O 1s binding energy is unchanged in both crystalline polymorphs but the binding energies of Ge 3d differ by 0·7±0·1 eV. Symmetric O 1s peaks from samples with x≤0·2 are assigned to bridging oxygens connecting combinations of 4 and 6 coordinated Ge. The O 1s spectrum from the x=0·3 sample contains contributions from bridging and nonbridging oxygens. Ge 3d spectra from the glasses have been fitted to contributions from 4 and 6 coordinated Ge giving values for N₆ (the fraction of 6 coordinated Ge) consistent with the theoretical relationship N₆=x(1-x)^-1 for x≤0·2. For the x=0·3 sample N₆ is lower than predicted due to replacement of 6 coordinated Ge by 4 coordinated Ge plus nonbridging oxygens. This lower value of N₆ would require there to be 20±3% nonbridging oxygens present and this is reasonably consistent with the value of 25±2% obtained by fitting the O 1s spectrum.